As shown in FIG. 1, known measurement methods of interfacial tension can be placed in five groups:                direct measurement using microbalance;        measurement of capillary pressure;        analysis of capillary gravity forces;        gravity distorted drop; and        reinforced distortion of drop.        
In the first method, surface tension is measured directly by a force sensor. Such systems use a plate or a ring of platinum-iridium alloy or platinum. The plates and rings have of standard dimensions, thus no calibration is required. In the second method, surface tension is proportional to capillary pressure, which can be measured directly with a pressure sensor. The third method measures gravity rise or size of a droplet after detachment. In the fourth method, the shape of the droplet is distorted by surface tension and gravity. Measuring the geometry of a pendant drop allows the determination of surface tension. For this measurement method, a CCD camera and computer evaluation is needed. The spinning drop technique evaluates the distortion of a drop and needs a CCD camera.
Besides these techniques, there is interest in interfacial tension measurement of small samples. The study of interfaces of very small particles and in finely dispersed systems is micro tensiometry. The main application fields of micro tensiometry are criminology, biology and pharmaceutical micro reactors. The two methods currently known for micro tensiometry are:                (a) micropipette technique; and        (b) atomic force microscopy.        
These are shown in FIG. 2.
In the micropipette technique of FIG. 2(a), a droplet is first captured at the tip of a micropipette. Utilizing the radian of curvature on both sides of the droplet as shown in FIG. 2(a)A, the surface tension can be calculated. This technique requires a microscope and an image recording system. The second approach of direct force measurement as shown in FIG. 2(a)B. A force sensor is again required.
A miniaturized version of the direct measurement method depicted in FIG. 1 is the use of atomic force microscopy to determine extremely small forces (FIG. 2(b)). The deflection of the micro cantilever is measured with a laser beam. Forces of the order of 1 pN can be measured. It has been proposed to use bubble generation and surface tension evaluation. The bubble is generated by electrolysis and detected electronically. The frequency of bubble formation is a measure surfactant concentration. A multi-well plate reader may be modified to evaluate surface tension. This technique utilizes the radius of curvature of the liquid surface acting as a fluidic lens and requires a camera system and an expensive commercial plate reader system.
All micro tensiometry techniques above require individual handling of a single droplet. As such, evaporation is a problem. Furthermore, the measurement is expensive and requires dedicated equipment. The bubble generation system is limited by the gas/liquid system of an aqueous sample.
It would be of advantage to be able to measure interfacial tension of micro droplets and bubbles in a simple configuration utilizing microfluidic technology. This should enable:                a small sample size, higher accuracy, and faster results;        interfacial tensions of all immiscible systems (both liquid/liquid and gas/liquid);        lower cost and easier handling        be suitable for hand-held systems and portable field measurements; and        an integrated “lab-on-chip” device with a microchannel and optical wave guides is possible.        